1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2013-186998 describes a spark plug for an internal combustion engine, which is configured to generate a spark discharge between its cylindrical ground electrode and center electrode when a high-frequency voltage is applied to the center electrode. This spark plug has the structure in which a cylindrical insulator is disposed such that the distal end thereof projects into the inside of the cylindrical ground electrode, and the distal end of the center electrode projects into the inside of the cylindrical insulator.
In this spark plug, when a high-frequency voltage or a pulse voltage is applied to the center electrode, a streamer discharge is generated in the beginning so as to cover the surface of the insulator mainly from the ground electrode. Thereafter, the streamer discharge spreads toward the center electrode, as a result of which a discharge path is formed between the center electrode and the ground electrode, and a glow discharge or an arc discharge is generated. An air-fuel mixture is ignited by this discharge. In the following, the word “discharge” means not a streamer discharge but a glow discharge or an arc discharge unless otherwise noted.
If the generated discharge keeps covering the surface of the insulator, since the cooling loss is large and accordingly a flame does not spread sufficiently, the ignitibility is low. Accordingly, it is required that the generated discharge is caused to detach from the surface of the insulator and to spread into the air by an airflow within a combustion chamber. To spread the discharge by an airflow sufficiently, it is necessary to mount the spark plug on an internal combustion engine such that the position of the discharge relative to the insulator and the direction of the airflow are in an appropriate relationship.
However, each of the ground electrode, the insulator, and the center electrode of the spark plug described in this patent document has a shape uniform in the plug circumferential direction. Accordingly, the position at which a discharge starts to occur is not determined to any specific circumferential position of the spark plug. That is, since the discharge start position is random, it is not possible to cause a generated discharge to spread stably in whichever direction the spark plug is oriented relative to the direction of the airflow within the combustion chamber.